The present invention relates to an automatic gain control system and more particularly to an automatic gain control system for a High Definition Television.
Currently, there are two ways to transmit data to a television (TV) receiver, the terrestrial transmission for the conventional TV and the cable transmission for Cable TV. Particularly, both the terrestrial and cable transmission use the vestigial sideband (VSB) modulation, by which the data is transmitted by multilevel symbols. The established standard data levels for the VSB signals are 2VSB with 2 symbol levels, 4VSB with 4 symbol levels, 8VSB with 8 symbol levels, and 16VSB with 16 symbol levels. The data level with the higher symbol levels can transmit greater data. However, as the data level increases, the number of symbols and the interspace between the symbols correspondingly decrease, resulting in a low signal to noise ratio (SNR). Particularly, the 16VSB with 16 symbol levels has the smallest interspace for each symbol and thus the lowest SNR.
The High Definition TV (HDTV) follows the suggested terrestrial and cable broadcasting standard using a VSB transmission of the 2, 4, 8, and 16 VSB. Accordingly, FIG. 1 shows a general block diagram of a HDTV incorporating an automatic gain control (AGC) system and FIG. 2 shows a block diagram of the AGC circuit.
Referring to FIG. 1, a HDTV automatic gain control system includes an antenna 10 receiving the transmitted signals; a tuner 12 tuning and converting the transmitted signal to an intermediate frequency (IF); an IF/carrier recoverer 14 converting the intermediate frequency to an analog-based signal, adjusting the gain factor according to the gain signal received, and outputting a gain control signal to the tuner 12; an A/D converter 16 converting the analog-based signal to a digital signal; and an AGC 18 determining the gain factor from the digital signal and outputting a gain control signal to the IF/carrier recoverer 14.
As shown in FIG. 2, the AGC 18 further includes an operator 180 taking the absolute value of the digitized signal and outputting the positive value, an adder 182 which sums the outputs from the operator 180 and the data level, an integrator 184 integrating the output of the adder 182, and a gain selector 186 which outputs a positive gain control signal if the integrator 184 outputs a negative value and outputs a negative gain control signal if the integrator 184 outputs a positive value. The adder 182 receives the positive signal value and assigns a negative polarity to the data level and outputs the summed value of the positive and the negative value.
In operation, the tuner 12 receives the input signal from the antenna 10 and converts the signal to an intermediate frequency. The IF/carrier recoverer 14 receives the intermediate frequency, converts the frequency signal to an analog-based signal, and outputs the analog-based signal to the A/D converter 16. The IF/carrier recoverer 14 also increases or decreases the gain according to the gain control signal from the AGC 18. The A/D converter 16 digitizes the analog based signal and outputs a digital signal to the AGC 18, wherein the absolute magnitude of the digital signal is taken and output to the adder 182.
In the signal transmission for a HDTV, every data line includes a segment synchronization (sync) signal in the first four symbols and the segment sync signal carries the designated data level with respect to the VSB mode. The adder 182 assigns a negative value to the designated data level and sums the positive digital signal value received from the operator 180 and the negative data level value of the segment sync signal. The integrator 184 receives the summed value from the adder 182, continually integrates the summed value, and outputs the integrated value to the gain selector 186.
If the integrated value is negative, the current gain on the signal transmission is determined to be lower than the data level and the gain selector 186 outputs a positive gain control signal to the IF/carrier recoverer 14. On the other hand, if the integrated signal value is positive, the current gain on the signal transmission is determined to be higher than the data level and the gain selector 186 outputs a negative gain control signal to the IF/carrier recoverer 14. The IF/carrier recoverer 14 receives either the positive or negative gain control signal from the gain selector 186 and accordingly adjusts the intermediate frequency. However, the IF/carrier recoverer has a predetermined range for adjusting the intermediate frequency and if the range were to be surpassed by the necessary adjustment, the control of the gain on the intermediate signal is transmitted back to the tuner 12.
Thus the gain of the analog based signal is converted by A/D converter 16 to a digital signal and the digitized signal level can be adjusted to the correct data level through the AGC 18. Up to now however, the automatic gain control system was directed to simply correct the data level without taking into consideration the number of symbol levels. Accordingly, the signal to noise ratio (SNR) suffers when the number of symbol levels increases because the interspace between each symbol level must necessarily decrease. One way to solve this problem is to use greater transmission power to maintain channel signal separations when the number of symbol levels increase. However, higher transmission power requires a greater cost.